Nearley – parser toolkit for JavaScript

Parsers turn strings of characters into meaningful data structures (like a JSON object!).nearley is a fast,feature-rich, and modern parser toolkit for JavaScript. nearley is an npm Staff Pick.

nearley 101

1. Install: $ npm install -g nearley
2. Write your grammar:

   # Match a CSS color
   # http://www.w3.org/TR/css3-color/#colorunits
   
   @builtin "whitespace.ne" # `_` means arbitrary amount of whitespace
   @builtin "number.ne"     # `int`, `decimal`, and `percentage` number primitives
   
   csscolor -> "#" hexdigit hexdigit hexdigit hexdigit hexdigit hexdigit
             | "#" hexdigit hexdigit hexdigit
             | "rgb"  _ "(" _ colnum _ "," _ colnum _ "," _ colnum _ ")"
             | "hsl"  _ "(" _ colnum _ "," _ colnum _ "," _ colnum _ ")"
             | "rgba" _ "(" _ colnum _ "," _ colnum _ "," _ colnum _ "," _ decimal _ ")"
             | "hsla" _ "(" _ colnum _ "," _ colnum _ "," _ colnum _ "," _ decimal _ ")"
   
   hexdigit -> [a-fA-F0-9]
   colnum   -> int | percentage

3. Compile your grammar:

   $ nearleyc csscolor.ne -o csscolor.js

4. Test your grammar:

   $ nearley-test -i "#00ff00" csscolor.js
   Parse results: 
   [ [ '#', [ '0' ], [ '0' ], [ 'f' ], [ 'f' ], [ '0' ], [ '0' ] ] ]

5. Turn your grammar into a generator:

   $ nearley-unparse -n 3 csscolor.js
   #Ab21F2
   rgb  ( -29.889%,7,8172)
   #a40

6. You try it! Type a CSS color here:

   …and the parsed output will appear here!
   

7. Create beautiful railroad diagrams to document your grammar formally.

   $ nearley-railroad csscolor.ne -o csscolor.html

   Output looks like:
   

   See a bigger example here.

Features

• nearley is the first JS parser to use theEarley algorithm (insert your own ‘early bird’ pun here). It also implements Joop Leo's optimizations for right-recursion, making it effectively linear-time for LL(k) grammars.
• nearley lives happily in node, but doesn't mind the browser.
• nearley outputs small files. And itsexpressive DSL comes with plenty ofsyntactic sugar to keep your source files short. And sweet.
• nearley's grammar language is powerful and expressive: you can use macros, import from a largebuiltin library of pre-defined parser-pieces, use a tokenizer for extra performance, and more!
• nearley is built on an idiomatic streaming API. You even have access to partial parses to buildpredictive user interfaces.
• nearley processes left recursion without choking. In fact, nearley will parse anything you throw at it without complaining or going into a sulk infinite loop.
• nearley handles ambiguous grammars gracefully. Ambiguous grammars can be parsed in multiple ways: instead of getting confused, nearley gives you all the parsings (in a deterministic order!).
• nearley allows for debugging with generous error detection. When it catches a parse-time error, nearley tells you exactly what went wrong and where.
• nearley is powerful enough to bebootstrapped. That means nearley uses nearley to compile parts of nearley. nearleyception!
• nearley parsers can be inverted to formgenerators which output random strings that match a grammar. Useful for writing test cases, fuzzers, andMad-Libs.
  
• You can export nearley parsers as railroad diagrams, which provide easy-to-understand documentation of your grammar.

Projects using nearley

• Artificial Intelligence and NLP: Shrdlite is a programming project in Artificial Intelligence, a course given at the University of Gothenburg and Chalmers University of Technology. It uses nearley for reading instructions in natural language (i.e. English).
• Standard formats: node-dmi is a module that reads iconstate metadata from BYOND DMI files, edtf.js is a parser for Extended Date Time Format, node-krl-parser is a KRL parser for node, bibliography is a BibTeX-to-HTML converter, biblatex-csl-converter converts between bibtex/CSL/JSON, scalpel parses CSS selectors, styled-components converts CSS to React Native.
• Templating and files: uPresent is a markdown-based presentation authoring system, saison is a minimal templating language, Packdown is a tool to generate human-readable archives of multiple files.
• Programming languages: Carbon is a C subset that compiles to JavaScript, optimized for game development, ezlang is a simple language, tlnccuwagnf is a fun general-purpose language, nanalang is a silly esoteric language, english is a less esoteric programming language, and ecmaless is an easily-extensible language.
• Mathematics: Solvent is a powerful desktop calculator, Truth-table is a tool to visualize propositional logic in truth tables, Emunotes is a personal Wiki with inline graphing and computation.
• Domain-specific languages: Hexant is a cellular automata simulator with a DSL for custom automata, Dicetower is an advanced dice plugin for hubot, deck.zone is a language to create board games, website-spec is a tool for functional web testing.
• Parsing libraries: nearley is a parser toolkit for JavaScript. It has a nearley-based DSL to specify parsers.

Excited? Get started onGithub, visit us on npm, or play with the calculator demo for more action.